We have been working in the area of manufacturing components for gas turbine engines from silicon nitride. In such applications, it is desired to obtain the maximum strength from the silicon nitride material as that material is subjected to severe operating conditions as a component of the hot gas flow path of such a turbine engine. We have been attempting to manufacture complex shaped, high density reaction bonded silicon nitride articles for use, for example, as the rotor of a gas turbine engine.
The process of this specification is one by which high density silicon nitride articles of complex shape can be made. A complex shaped, reaction bonded silicon nitride article of moderate density can be made by methods already known in the art, such as injection molding and slip casting processes. By the process of our invention, the thus made reaction bonded silicon nitride article of moderate density is sintered so that the strength and density are increased. During the sintering, there is some slight shrinkage of the article, but not such a significant amount of shrinkage as will produce distortion, cracking, or other failures in the article.
By sintering, we mean the following. During a sintering and densification operation, the particles of a powder first form necks between adjacent particles which become the grain boundaries. Between the grain boundaries, a liquid phase may exist. The densification proceeds by a reduction in the number and size of the spaces or pores between the particles as they migrate into these grain boundaries through a transport mechanism which is followed by a partial collapse or moving together of the center points of the grains, thus producing densification.
In the method of this invention, we can tolerate a small degree of shrinkage of the article from its as-formed, low density state to its densified state. By its as-formed state, we mean a reaction bonded silicon nitride article which has been made through a process in which materials such as silicon particles are slip cast to form a particular configuration and thereafter nitrided, or wherein materials such as silicon particles are injection molded with a suitable binder, subjected to a pyrolizing operation, and thereafter nitrided. The article is not a reaction bonded silicon nitride article until after the silicon particles have been nitrided in a nitriding operation. By using either slip cast or injection molding techniques, we can make an article of complex configuration such as a rotor for a gas turbine engine. By knowing that the rate of shrinkage is going to be controlled, we can specifically make the asformed article larger than desired so that when the densification and associated shrinkage takes place, the fully densified article will have the dimensions desired in the final article, or be so close thereto that they may be readily machined to the final desired dimensions.
The prior art search conducted on the subject matter of this invention did not uncover any patents which we felt disclosed or anticipated the method of densification set forth in this specification. However, two patents did appear of interest, namely, U.S. Pat. Nos. 3,992,497 and 4,038,092. These patents will be discussed below.
U.S. Pat. No. 3,992,497 discloses a method for making a silicon nitride article in which a silicon nitride powder, not silicon powder, is mixed with a densification aid and heated rapidly to a sintering/densification temperature in the range of from 1500.degree. C. to 1700.degree. C. and held there for a very short period of time in the range from 5 to 30 minutes. Thereafter, the product is rapidly cooled. The patent did not state the particular atmosphere used during the sintering operation. This disclosed process is different from the one disclosed herein in that the temperatures are significantly lower than those used by us, the time at temperature for the sintering operation is very much less than those required in our process and there is no stated requirement that a nitrogen atmosphere be present, as is required in our process. As an additional matter, when silicon nitride powder, rather than the silicon powder we start with, which is subsequently converted into silicon nitride after being formed into the shape of an article, is mixed with a densification aid and thereafter formed into an article which is subsequently sintered, a great deal of shrinkage takes place in the article, e.g., as much as 20% or more.
U.S. Pat. No. 4,038,092 discloses a process for increasing the strength of a reaction sintered silicon nitride body by impregnating the surface of that body with a finely divided alumina. After such impregnation, the article is fired in a nitrogen atmosphere to a temperature in excess of 1200.degree. C. up to about 1400.degree. C. The patent also states that this reaction is carried out under an appreciable pressure of silicon monoxide so that disassociation of silicon nitride and a loss of silicon from the body is avoided. This patent does not teach a sintering of a body, but rather teaches the formation of a surface layer in which the alumina plays a part. Also, the patent states that the use of a silicon monoxide atmosphere is absolutely essential to the process. The process taught in this patent is different than the one to be disclosed in this specification in that the temperature limits are very low, the requirement of a silicon monoxide atmosphere being present during the action, and the fact that alumina is used only to aid in the formation of a surface layer on the article.